Radiation sources emitting EUV radiation and/or soft X-rays are in particular required in the field of EUV lithography. The radiation is emitted from a hot plasma produced by a pulsed current. The most powerful EUV lamps known up to now are operated with metal vapor to generate the required plasma. An example of such an EUV lamp is shown in WO2005/025280 A2. In this known EUV lamp, the metal vapor is produced from a metal melt which is applied to a surface in the discharge space between the electrodes and at least partially evaporated by an energy beam, in particular by a laser beam. In a preferred embodiment of this EUV lamp, the two electrodes are rotatably mounted, forming electrode wheels which are rotated during operation of the lamp. The electrode wheels, during rotation, dip into containers with the metal melt. A pulsed laser beam is directed directly to the surface of one of the electrodes in order to generate the metal vapor from the applied metal melt and ignite the electrical discharge. The metal vapor is heated by a current of some kA up to approximately 10 kA, so that the desired ionization stages are excited and radiation of the desired wavelength is emitted.
A common problem of known EUV and/or soft X-ray lamps is that the efficiency of the conversion of supplied electrical energy into EUV radiation and/or soft X-rays of a desired small bandwidth is low. In particular in the field of optical lithography for the semiconductor industry, EUV radiation around 13.5 nm within a 2% bandwidth is required.